Direct communications and cellular communications have fundamental differences. Cellular communications would apply power control for all communication devices attaching to the network. When all communication devices connect to a base station (BS), the BS generally applies power control mechanism to adjust transmission power of each communication device. In other words, all communication devices could be served fairly. However, transmission power of direct communication is not centrally-coordinated. In order to increase successful network attachments or network associations or transmissions for direct communications, transmitting largest power may be the best policy. Especially for a random access procedure, a communication device may have no prior knowledge of a destination device, and the communication device would then transmit the largest power in the beginning to acquire the best chance of successful transmission. When all communication devices transmit at the largest power at the same time, all communication devices would collide together, and such situation may result in failed transmissions and failed associated frequency resources. In general, the larger the transmission power, the more communication devices are influenced, but the spectrum efficiency would be less.
On the other hand, the transmission range in a direct communication may influence the reception performance of a victim device or the overall direct communication system capacity. The larger the transmission range, the less direct communication devices could use identical time and frequency region simultaneously.
Direct communications would generally include two phases—a random access phase and a transmission phase. In a general direct communication process, the purpose of the random access phase is to build a link between a communication device and an access point, e.g. WiFi.
During the random access phase, a communication device would send a signal to an access point to content for its resource with other communication devices. The access point may acknowledge the communication device attached to the access point. The access point may further send the associated resource for a communication device to send messages to be followed. In a general direct communication process, the purpose of the transmission phase is to exchange messages between the communication device and the access point. The communication device would transmit data to the access point. The access point may acknowledge the communication device by sending the communication device data to be received.
During the random access phase, all communication devices content for similar transmission power and without priority. Multiple communication devices may content at the same time for frequency and time resources. If a communication device applies a lower transmission power, the communication device would have a less successful rate of transmission. As such, there would be some communication devices which can not content. Therefore, without predefined protocol(s), all communication devices would transmit at the largest power in order to reach an equal rate of successful transmission. Transmission in the largest power may imply less frequency re-use in the frequency domain, which further implies inefficient spectrum usage. As such, the conventional approach works only for a very small number of communication devices, e.g., only 10 communication devices in a WiFi network. However, such a small number of participating communication devices in the direct communications is not sufficient to support direct communications in a longer range, such a 1 km radius.